Laser receivers are often used at construction sites in order to locate a reference laser beam, in particular the laser beam of a rotating construction laser or the fan like laser beam of a line laser. Rotating construction lasers, which are designed to provide a laser light plane upon rotating the emitted laser beam are well known by persons skilled in the art and therefore not described in detail herein. An embodiment of a line laser generating a fan like laser light plane is i.e. given in EP 1988360 A1. The reference light plane generated by the rotating construction laser or the line laser may be horizontal or inclined. It is used at the construction site for various purposes, i.e. for guiding construction machines or for carrying out various measurements based on the plane, i.e. for allowing construction workers to quickly lay out visible marking-lines or adjust heights.
An example for a laser guided construction machine is given in U.S. Pat. No. 6,691,437 B1. Disclosed in this document is a level sensing system for use with an excavating machine. A laser receiver is mounted on the dipper stick of the machine in order to indicate the relative location of the dipper stick to the reference laser beam, based on the incidences of the laser beam striking over the light receiving sensor of the laser receiver. The dipper stick is further provided with an inclinometer, which inclinometer comprises a gyroscopic inclination sensor and an accelerometer. The accelerometer senses acceleration due to gravity and provides a static vertical reference in order to compensate the long term drift of the gyroscopic inclination sensor.
However, due to eye safety regulations laser emitters used on a construction site, i.e. rotating construction lasers and line lasers have to have low power beams. Hence, the laser beam of such a conventional laser emitter is—on the one hand —able to indicate, i.e. a precise visible reference line at a wall near by, but—on the other hand—tends to defocus and become dimmer at further distance to the light source, so that the beam has to be located by using a laser receiver.
Conventional laser receivers include a laser light photo sensor and a circuitry, wherein the photo sensor comprises a linear array of photo sensitive elements and is connected to said circuitry. The photo sensitive elements provide an electrical output signal, when illuminated by the laser beam, which electrical output signal is transferred to and computed by the circuitry. The results of the computation are presented to the user by output means like a display integrated in the laser receiver and connected to the circuitry. Examples of such laser receivers are given, i.e. in EP 2 199 739 A1, U.S. Pat. No. 7,409,312 B and WO 2008/008210 A2. Particularly, those laser receivers have a defined zero position, e. g. the centre of the photo sensor array. For localization of the laser beam, the zero position of the laser receiver has to be brought steadily into match with the laser plane. Some laser receivers, like the laser receiver disclosed in U.S. Pat. No. 7,409,312 B, are further provided with a gravity reference device and optionally with a GPS receiver in order to facilitate a precise localization. An additional laser distance measurement device may further allow for computing a 3D position of the laser receiver. The method described in WO 2008/008210 A2 discloses to integrate a rangefinder within the laser receiver for determining the distance to the laser emitter. However, this requires that the laser receiver orientation is very well aligned to the laser emitter, which is often quite difficult especially at construction sites.
Precise localization of the reference laser light plane is a time consuming procedure, especially when it is a laser light plane generated by a rotating laser beam, in particular if an infrared laser beam is used. The conventional hand-held laser receiver has to be swept several times slowly in a direction perpendicular to the laser light plane in order to catch at least two strikes of the laser beam with each of the sweeps, i.e. a first strike at the outermost photo sensitive element of the linear array of photo sensitive elements and a second strike at a photosensitive element closer to the zero position in the center of the linear array of photo sensitive elements. The different incidences at the array of photo sensitive elements indicate the moving direction of the laser receiver with respect to the laser light plane, which moving direction is derived by the circuitry from the electronic output signals of the photo sensitive elements and is indicated to the user by an according output, i.e. on the display of the laser receiver or as an audio signal. In order to bring the laser receiver in a position, where consecutive strikes of the laser beam illuminate only the zero position at the center of the aligned photo sensitive elements, so that the laser receiver is on-grade, the user moves the laser receiver in an iterative process of consecutive sweeps through the laser light plane. Thus, it can be stated that to know the laser light plane and/or the position and orientation of the laser receiver relative to the laser emitter on a construction site is still a time-consuming procedure.